Does having ‘Good Genes’ make you sexy?



  • Natural Selections rule #4:
    • If variation in a heritable trait is associated with variation in reproductive success…
    • those traits accumulate in population


  • Brainstorm: What traits might be associated with mating decisions between males and females in the animal kingdom?


  • Natural selection is way broader than you thought!
    • Darwin proposed models of sexual selection

Sexual selection is one kind of natural selection



  • Selection can occur as an association between phenotype and mating success
    • secondary traits in mating
    • Sexual Dimorphism = males and females look different


  • Introduces choosiness into evolution and reproduction
    • ‘mate choice’


  • Often: male ‘showiness’ in nature
    • benefits and risks of showiness…

Is this why males look and behave differently?

What are these male traits ‘expressing’ to females?
















If heritability > 0, then traits that increase reproductive success should accumulate

Intrasexual selection is the battleground for mating



  • ‘Choosiness’ drives competition
    • usually between males


  • Occurs in many species
    • Direct combat
    • Subtle combat
    • Display (rituals and colors)


  • Drives the evolution of secondary sexual characteristics

Direct Combat: Stag beetles (Goyens et al. 2016)

Subtle combat


  • Often post-copulatory sperm competition
    • selection of diverse and elaborate male genitalia


  • Common in insects
    • Piskin and Koskin 2010
  • Common in birds


  • “Many bizarre traits, including male genitalia that explode during copulation, male genitalia that “sing” during copulation, potent seminal products that invade the female’s body cavity and her nervous system to influence her behavior…. Eberhard 2009”

Subtle combat

Male displays and rituals (superb lyrebird)

Male displays and rituals (birds of paradise)

Male displays and rituals (widowbird)

Male displays and rituals (satin bowerbirds)

Male displays in birds….

Why is there sexual dimorphism?


  • Why be choosy?
    • what are the real rewards for the female?


  • How common is choosiness??


  • Sexual selection results in the evolution of sexually dimorphic traits
    • preferential mating
    • direct fitness benefit
    • indirect fitness benefit


  • Heterosexual conversation….for now

Female guppies prefer males with orange spots….Why?

Female blackbirds prefer males with orange beaks.. Why?

Brainstorm: Why do animals prefer symmetric partners?

Runaway sexual selection

Breaking the binary

  • Sex is not just heterosexual in animals
    • ~1,500 species and counting….
    • dragonfly, dolphin, elephant, orcas, turkey…
    • beetles, sheep, bats, orangutans….
    • most present in social species
    • animals have collections of sexual behaviors


  • Gender binary doesn’t exist in nature
    • if gender exists in nature, it is very fluid
    • sexual mimicry & masking sexuality


  • Intersexual animals
    • female -> male, male -> female
    • female and male at same time
    • male seahorses can get pregnant
  • So is sexual selection a thing?
    • more complicated than it seems on the surface